Add the marinara to the base of a large baking dish, about 30x25 cm (12x10 in inches). Set aside.
400 grams marinara (tomato sauce)
Start the water for the pasta. Once the water boils, cook the shells in well salted water according to package directions. Cook for two minutes less than is stated on the package.
200 grams conchiglioni
In the meantime, prepare the filling. Heat a large frying pan over medium heat, then add the olive oil.
2 teaspoons olive oil
Add the mushrooms to the pan and spread into an even layer. Cook, avoiding stirring, for 3-4 minutes, or until browned.
300 grams oyster mushrooms, finely chopped
Add the onion and cook for another 1-2 minutes. Add the garlic, salt, and pepper, and cook for another minute.
1 small onion, 3 cloves garlic, 1/2 teaspoon sea salt, 1/2 teaspoon black pepper
Take the pan off the heat and add the baby spinach. Stir to wilt the spinach with the residual heat from the pan.
100 grams baby spinach
Add the cashew cream cheese and stir through. Set aside until the pasta is ready.
150 grams cashew cream cheese
Drain the pasta (don't rinse it). Working carefully, fill each shell with a heaping tablespoon of the filling. Place each filled shell into the tomato sauce in the baking dish, repeating until all of the shells are filled.
Bake for 10 minutes. After this time, the pasta should be a bit bigger, but not browned.
Remove from the oven, add any toppings you'd like, and serve immediately. Leftovers keep well for a day or two in a sealed container in the refrigerator.
Notes
This can serve up to four people, if served with sides. We couldn't finish the whole dish, so as a main without anything else, I'd say it serves two very generously or three normally.